Aircraft Video GPS
Jim Vosbourgh Terra Surveys Limited, 1962 Mills Road, Sidney, BC, Canada V8L 5Y3 Ph:(250)656-0931 Fx:(250)656.4604 E-mail: terra@sidney.terrasurveys.com? Ian Scott West Kootenay Power, 1290 Esplanade, P. O. Box 130, Trail, BC, Canada V1R4L4 Ph:(250)368-0524 Fx:(250)368-8293 E-mail : iscott@wkpower.com
Abstract
While utilities struggle with which platform to choose and justify the dollars necessary to implement it in hardware and software, the often underrated item is the effort required to gather the vast amounts of data the new system will consume. Probably the main reason for moving to an AM/FM/GIS system in the first place is that the current records are sadly out of date and inadequate. The challenge is to find and accurate an cost effective way of updating the new system when the old records cannot be relied on. This paper describes the successful use of videographic mapping to capture right of way features. In 1995 West Kootenay Power was well under way in developing a AM/FM/GIS Autocad based system. Also there had been a growing need for a comprehensive vegetation management program over the previous decade. Most of the utility corridors cut through forested country. The cost and liability of hazard tree removal and brushing (undercover suppression) were rising annually. Right at the outset it was determined that we wanted as much of the brushing inventory to be captured in a single year; in effect, to get a snapshot of the corporation’s field assets at a given time. It followed from this that the only way to capture most of the some 3,000 kilometers of rural right-of-way was to use Global Positioning System (GPS) technology. We had some experience with the capabilities of GPS, but only as used by ground surveyors. We remembered seeing a presentation in which GPS had been linked to a vertically mounted video camera in a small aircraft. The combination of(a) combining digital images and GPS together, and (b) using a light helicopter instead of a large twin-engined plane, had struck Scott as a possible alternative to the time-consuming methods of traditional ground surveying. Testing for accuracy - how good is ‘GOOD’? Would an airborne GPS and video system be cost competitive? And more importantly, could it match the required ground accuracies of+ 1 meter which were needed, and which were attainable using conventional ground survey methods? Enquiries resulted in two airborne GPS contractors reaching the shortlist. The first used a helicopter which hovered over each pole, while the operator took a fix on the pole with something akin to a bomb-sight. The fix and the DGPS position were logged, and the helicopter moved on to the next pole. Given the mountainous terrain of the West Kootenay Power territory (the interior of British Columbia has valleys as low as 100m (300 ft) above sea level and peaks of over 2,500m (8,000 ft)), and the cost of this stop-start method, this option was discounted. The alternative used a videographic technique to map the poles. Simply put, a vertically mounted video camera replaced the traditional aerial camera (with its expensive large-format film), while a sophisticated positioning system ran in the background. Later, the image data and the positional data were merged, so any video frame could be used to map the terrain. In this process, many of the time-consuming and costly steps used in conventional airphoto (the field survey, developing & printing photos, aerial triangulation, photomosaicing and photogrammetry) were eliminated. But was it accurate enough? Could a small helicopter, bouncing through mountain turbulence, really be used to position slender power poles to sub-meter accuracy, while flying at 100 krn/hr (60 mph)? The obvious answer was to put the videographic system (VideoMap) to a full blind test, to see if it could deliver. Ian Scott reached an agreement whereby if VideoMap could produce the claimed sub-meter accuracy, the developer/operator (Terra Surveys Ltd of Victoria, BC) would be awarded the contract to map the rural sections of the WKP transmission system. If the accuracy was not met, the operator agreed to pay half of the helicopter costs for the trial. VideoMap was flown along a 10 km test corridor, where a number of pole positions had previously been determined by a differential GPS ground survey. The results met West Kootenay Power’s criteria so convincingly that Scott requested the company provide all the remaining pole positions on the test line, to serve as benchmarks when evaluating the accuracies of the ground crews to be used in the urban areas ! Subsequently, both WKP’s transmission and much of their rural distribution network, spanning an area from Princeton, BC in the west, to Creston in the east, was flown in 7 days. The survey identified and positioned over 34,000 poles in approximately 3,200 km (2,000 miles) of flying. Each pole was described by casting, northing and height above sea level, plus up to three different descriptive categories were included per pole. Table 1 shows typical text descriptions.
The New Technology - VideoMap How does videography work? Many people think that flying with a video camera hanging out of the window is videography. Not so. First, the exact position of the aircraft is monitored and recorded continuously using DGPS. But this only tells you where the antenna on the aircraft is. Next, the offset between the antenna and camera is added. Now we know where the camera is. But the aircraft can rotate in 3-dimensional space, so an inertial navigation system (INS) in the form of a ring-laser gyro is mounted in the airframe. This instrument records the heading, pitch, and roll of the aircrafl. Now, simple geometry allows us to know where the camera is pointing. But it still does not allow us to determine the scale of the image on the ground. Are we low (at 100m above the ground) with a small field of view, or are we high (2,000m) with a wide view? A laser profiler fires between 10-100 pulses per second to measure the range between the aircraft and the ground. With this piece of the puzzle known, geometry allows the image seen on the video to be automatically scaled and referenced in 3-D space. Finally, with the power of modern desktop computers, all this data can be collected and processed at very price competitive rates. In most applications, two cameras are used in the aircraft. The nadir unit faces straight down, and is the one used for mapping. A second unit faces obliquely forward, and allows the operator to see what is coming up, before it appears in the nadir field of view. The cameras record 30 frames/second, which can be analyzed later individually, or analytically in stereo pairs or multiples. Because the process works in the digital world (rather than the traditional optical one), the computer calculates heights and slopes, rather than relying on an operator/viewer. The video tapes may be viewed on a conventional television screen or a PC monitor, to ‘get a feel’ for the country. This is ‘subjective viewing’. When used in conjunction with the INS and DGPS data in VideoSoft, an operator can determine exact coordinates for any point appearing in the video frame. The benefits of using video The realm in which VideoMap is particularly strong is in the mapping and positioning of linear features such as rights-of-way, roads, rivers, and, on a larger scale, valley systems. Any linear ground feature lends itself to this style of mapping. Scale and resolution are controlled by the flying height and camera lens. If small scale features such as roads or streams are to be mapped in detail, the aircraft flies lower, with a reduced swath width. Higher altitudes provide ‘the big picture’, but with reduced resolution. For example, a 0.2m (8”) pixel resolution is attainable with a flying height of 300m( 1000’), while 2m (8’) pixel resolution would be achieved froma3000m(10000’) flying height using the same lens system. What do you want to see—an insulator on a pole, or just the pylon? A Stand-Alone Platform: One of the time-consuming phases in aerial mapping is determining where you are, when the photograph you are looking at has no identifiable feature (such a road junction, river bend, etc). The standalone capability of VideoMap means no such ground registration is required to reference the data to UTM or Latitude/Longitude. A single point reference is used to post-process the differential GPS data sets. Cost and time savings are considerable. Data for a major contract can be collected in a matter of hours or days, as opposed to weeks or months using traditional surveying methods. This is particularly true where the terrain is rough. Field workers and associated high operational costs are eliminated, while the quality and quantity of data collected can be equal to or superior to that collected with traditional ground survey methods. High Ground Accuracy: Because both the small aircraft and video tape are cheap, the operator can afford to fly low and cover large areas in great detail. As a result, the accuracy on the ground is much improved. Registration of power poles is possible in the 35-50cm range, and height accuracies of 50-75cm are normal. Reduced Time and Cost: The cameras are small and light, and can fit into a small fixed wing aircraft (a Cessna 182 is routinely used) or helicopter (an A-STAR was used for the WKP project). The unit also fits into a custom pod which attaches to the underside of a Bell 206. VideoMap eliminates the need to fly traditional photography, provide ground control, or photogrammetrically transform the photo to a digital image. VideoMap is already a digitally gee-referenced image. Wide weather window: Any utility corporation knows the cost of aerial photography. What is not so obvious is the high cost of standby, when the aircraft is mobilized, but the cloud cover prevents flying. The standby costs are always an uncertainty, and can change the project costs significantly, based solely on luck (or lack of it). Enter videography: with its improved low light sensitivity, and ability to fly under most of the weather, videography requires much less stringent flying conditions, and can frequently get the project ‘in the can’ before the conventional airphoto plane can even get off the ground. Comparing Mapping Technologies How does videography compare to other mapping methods? What are the other conventional data acquisition methods, and the costs and benefits associated with each?
Output from the project WKP received a copy of the flight videos, plus digital data sets of all pole locations, on disk. The videos permit a different perspective of the mapped area. For example, depending on the client and their needs, they can be used by managers who are not involved with the T&D of the project, but who want to examine unrelated phenomena, such as rights-of-way encroachment, legal boundaries, or service road conditions. A ‘birds-eye view’ of the area is a powerful tool in the planning and decision making process. The Terra Surveys Ltd project produced:
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